thinkphp php生成六位随机字母

function create_invite_code(){
        $d = substr(base_convert(md5(uniqid(md5(microtime(true)),true)), 16, 10), 0, 6);
        $w['referral_code'] = array('eq', $d);
        $user_info = Db::name('user')->field("id")->where($w)->find();
        if ($user_info) {
            $this->create_invite_code();
        }

        return $d;
}